Measurements of Some Volatile Compounds by Means of the Electronic Nose

Using an electronic nose, concentration ranges of volatile fatty acids(VFAs), methane and butane, NH3, HCl, SO2 andN2O have been measured to establish the relation between theconcentration in the liquid or the gas sample and the electronic nosereading. A quantitative sensorial odor perception (SOP) was introduced,based on the average reaction of the twelve available sensors of theelectronic nose. The results of the different compounds showed that thesensors reach a saturation level with increasing concentration. In the lowerconcentration ranges, linearity between concentration and signal outputoccurred. This linear interval was situated for the VFAs between thedetection limit in the range of 5 to 15 g dissolved compound per L distilledwater and the upper limit of 60 g L-1. For the gases, thedetection limit varied between 6 and 690 volumes of gas per million ofvolumes air (ppmv). The upper limit of the linear interval ranged from 100–3000 ppmv depending on the compound. For the olfactometry reference product n-butanol, with a reported olfactory lower threshold valueof 0.04 ppmv, the electronic nose was less sensitive and gave a detectionlimit around 975 ppmv. The different compounds could be visualized in radarplots, which had a specific profile for each compound. The higher theconcentration of the volatile compounds in the air, the larger the surfaceof the respective radarplot. A discriminant analysis showed clusters ofcompounds such as the VFAs, the non polar gaseous compounds methane andbutane and the other more polar gaseous compounds.